The present invention relates generally to the field of flue gas cleanup apparatuses and, in particular, to a new and useful hybrid, wet electrostatic precipitator (HWESP) used to remove particulate and contaminants from exhaust gases.
Electrostatic precipitators (ESPs) are used in coal-fired power plants, the cement industry, mineral ore processing and many other industries to remove particulate from an exhaust gas stream. ESPs are particularly well suited for high efficiency removal of fine particles from a gas stream. Specially designed dry ESPs have attained particle collection efficiencies as high as 99.9%. However, conventional ESP collection efficiencies are at their lowest values for fine particle sizes between 0.1-1.0 microns. Additionally, conventional ESPs cannot address the problem of gaseous emissions or gas-to-particle conversion. The release of substances such as acid gases and mercury from the flue gas generated by the combustion of fossil fuel poses a major health concern and is regulated by law.
An ESP electrically charges the ash particles in the flue gas to collect and remove them. As shown in FIG. 1, the unit is comprised of a series of parallel vertical collection plates 5 through which the flue gas passes. Centered between the plates are charging electrodes 10 which provide the electric field. The collection plates 5 are typically electrically grounded and are the positive electrode components. The discharge electrodes 10 in the flue gas stream are connected to a high voltage power source, typically 55 to 75 k V DC average, with a negative polarity. An electric field is established between the discharge electrodes and the collecting surface. As the flue gas passes through the electric field, the particulate takes on a negative charge which, depending on particle size, is accomplished by field charging or diffusion. The negatively charged particles are attracted toward the grounded collection plates 5 and migrate across the gas flow. Some particles are difficult to charge, requiring a strong electric field. Other particles are charged easily and are driven toward the collection plates 5 but also may lose the charge easily requiring recharging and recollection. Gas velocity between the collection plates 5 is also an important factor in the collection process since lower velocities permit more time for the charged particles to move to the collection plates 5 and reduce the likelihood of re-entrainment. In addition, a series zones of collection plates 5 and discharge electrodes 10 are necessary to maximize overall particulate collection by increasing the opportunities of the individual ash particles to be charged and collected. The ash particles form an ash layer as they accumulate on the collection plates 5. The particles remain on the collection plate surface due to the forces from the electric field as well as the molecular, mechanical, cohesive, and adhesive forces between particles. These forces also tend to make the particles agglomerate or cling together. Examples of a typical ESP structure is shown and described in U.S. Pat. Nos. 4,276,056, 4,321,067, 4,239,514, 4,058,377, and 4,035,886, which are incorporated herein by reference.
The collection of acid mists, consisting of fine particulate, has been accomplished with wet ESPs in many industrial processes. These units differ from the dry, or conventional, ESPs in materials of construction and cleaning methods; however, the collection mechanism is basically the same. In wet ESPs, cleaning of the collecting plates is performed by washing the collection surface with liquid, rather than mechanically rapping the collection plates or utilizing sonic horns, as with dry ESPs. Reintrainment from the cleaning of the collection plate surface is generally not an issue in wet ESP. Wet ESPs use the wetting of the collection surface area to remove particulate from the collection plate, which drains into a hopper, trough, or pan. Because wet ESPs operate in a wet environment in order to wash the collection surface, they can handle a wider variety of pollutants and gas conditions than dry ESPs.
In most wet ESPs, both tubular and flat-plate, the collection surface normally is a plain, solid, continuous sheet of metal or plastic. Therefore, the flushing liquid passing over the surface tends to “bead” due to both surface tension effects, as well, as the geometric imperfections of the surface. Because the flushing liquid cannot be uniformly distributed over the surface, this beading can lead to channeling and formation of “dry spots” of collected particles. The resulting build-up of collected material can cause the ESP electrical performance to degrade because the accumulated material is not as good a conductor as the underlying substrate or the water. As a result, current flow is inhibited, which results in increased emissions.
A wet ESP's collection section can be made out of any conductive material. Wet ESP components have been made out of conductive fiberglass, carbon steel, various stainless steels and various high-end alloys. Non-conductive materials can also be used if the material is wetted to provide a means of surface conductivity. Wet ESPs fabricated with metal collecting plates may require expensive high alloy stainless steels to withstand corrosion from the various wet environments.
Pasic et al., in U.S. Pat. No. 6,231,643, which is incorporated herein by reference, first disclosed the principle of using a membrane as a collecting electrode in a dry or a wet ESP. However, the turbulent flow of gases around the membrane electrodes of Pasic et al. prevents substantial collection of acid aerosols and fine particulate.
The structure forming the sides and roof of an ESP is typically a gas-tight metal encased enclosure. The structure rests on a lower grid, which serves as a base and is free to move as needed to accommodate thermal expansion. All of the collecting plates and the discharge electrode system are top supported from the upper girder assemblies. Access doors in the casing and adequately sized walkways between the fields assist in maintenance access for the internals. Metal pyramid or trough shaped hoppers or pans are supported from the lower grid and are made of externally stiffened casing. Hoppers are generally designed as particulate and liquid collection devices.